1. Technical Field
The present invention relates to a method for producing an L-amino acid by fermentation, and more specifically to genes which aid in this fermentation. These genes are useful for improving L-amino acid production, particularly L-phenylalanine and L-histidine.
2. Background Art
Conventionally, L-amino acids are industrially produced by fermentation methods utilizing strains of microorganisms obtained from natural sources, or mutants thereof. Typically, the microorganisms are modified to enhance production yields of L-amino acids. Many techniques to enhance L-amino acid production yields have been reported, including transformation of microorganisms with recombinant DNA (see, for example, U.S. Pat. No. 4,278,765). Other techniques for enhancing production yields include increasing the activities of enzymes involved in amino acid biosynthesis and/or desensitizing the target enzymes of the feedback inhibition by the resulting L-amino acid (see, for example, WO 95/16042 or U.S. Pat. Nos. 4,346,170, 5,661,012 and 6,040,160) and creating bacterial strains which are deficient in the genes which use the precursors of the target compound for other pathways, or creating bacterial strains which are deficient in the genes responsible for degradation of the target compound.
These manipulations usually result in strains which cannot grow, grow only at a significantly reduced rate, or require additional nutrients, such as amino acids. For example, enhancing the expression of some genes may become excessive and could lead to significant inhibition of bacterial growth and, as a result, lower the ability of the bacterium to produce the target compound.
A possible approach for avoiding the difficulties described above is to optimize the expression of the genes which encode proteins involved in the distribution of the carbon, nitrogen, or phosphorus fluxes, or the excretion of the target substance out of the bacterial cell.
This aim was often accomplished by introducing a mutation into a promoter sequence of the amino acid- or nucleic acid-biosynthesizing genes (European Patent Application EP1033407A1), by obtaining the library of synthetic promoters with different strengths (Jensen P. R., and Hammer K., Appl. Environ. Microbiol., 1998, 64, No. 1. 82-87 Biotechnol. Bioeng., 1998, 58, 2-3, 191-5), and creating a library of artificial promoters (PCT application WO03089605).
It is known that tagging a protein with the ssrA peptide tag, encoded by SsrA RNA, marks the tagged protein for proteolytic degradation (Gottesmann S. and al, Genes & Dev., 12:1338-1347 (1998)). SsrA RNA (Synonyms: SsrA, tmRNA, 10Sa RNA, transfer-messenger RNA, SipB, B2621) functions to release a stalled ribosome from the end of a “broken” mRNA that is missing a stop codon by acting both as a tRNA and as an mRNA, using a “trans-translation” mechanism. SsrA also mediates the release of the stalled ribosome due to lack of an appropriate tRNA (Hayes C. S. and al, PNAS, 99(6):3440-3445 (2002)). Other translation problems can also lead to SsrA activity. In addition, SsrA RNA stimulates degradation of defective mRNAs (Yamamoto Y. et al, RNA, 9:408-418 (2003)).
The SsrA RNA contains a tRNA-like structural region that is processed by RNase P and charged by alanyl-tRNA synthetase (Komine Y et al, Proc. Natl. Acad. Sci. USA, 91 (20):9223-7 (1994)). The SsrA RNA also has a region that acts as a messenger RNA and encodes a translated tag that is added to the nascent protein and which targets this protein for degradation (Tu G.-F. and al, J Biol Chem, 270(16):9322-9326 (1995)). The structure of the SsrA RNA and the translational mechanism have been examined in detail (Corvaisier S. et al, J Biol Chem, 278(17):14788-97 (2003)). The SsrA RNA is transcribed as a larger precursor RNA that is then processed to form the mature RNA.
SsrA is similar to RNAs from Mycoplasma capricolum, Bacillus subtilis Ueda K. et al., Genes to Cells, 7(5): 509-519 (2000)), Dichelobacter nodosus, Synechococcus sp. strains PCC6301 and PCC6803, Thermus thermophilus, Salmonella enterica serovar Typhimurium, and to a two-piece RNA from Caulobacter crescentus. The tmRNA genes have been used as probes for the identification of bacterial species (Schonhuber W. and al, BMC Microbiology, 1(1):20 (2001)).
However, there have been no reports describing the use of ssrA-tagging for L-amino acid production, for example, L-phenylalanine or L-histidine production. Particularly, there is no previous description of using a bacterium of the Enterobacteriaceae family containing a DNA encoding the peptide of SEQ ID NO: 2 or a variant thereof, attached immediately downstream of a gene encoding a bacterial enzyme which influences L-amino acid biosynthesis.